Shroomish locations — where to catch

In Paldea (Scarlet & Violet)

• South Province (Area Five) — Walk
• South Province (Area Three) — Walk
• West Province (Area Three) — Walk

In Sinnoh (Diamond, Pearl & Platinum)

• Great Marsh — Walk

In Johto & Kanto (HeartGold & SoulSilver)

• Johto Safari Zone — Walk
• Viridian Forest — Headbutt

In Hoenn (Ruby, Sapphire & Emerald)

• Petalburg Woods — Walk

Other games (4)

• Brilliant Diamond / Shining Pearl (Gen 8): Great Marsh
• Omega Ruby / Alpha Sapphire (Gen 6): Petalburg Woods
• Black / White (Gen 5): Route 11
• XD (Gen 3): Cipher Lab

Shroomish — full Pokédex entry